The geology and geochemistry of the Naringla Monzodiorite ..._… · Vein Alteration 4.21 Goodrich...
Transcript of The geology and geochemistry of the Naringla Monzodiorite ..._… · Vein Alteration 4.21 Goodrich...
The Geology and Geochemistry of the Naringla Honzodiorite, and the mineralization and alteration of the associated Porphyry Copper Deposits, Yeoval, Central-Western N.S.W.
D. R. C 0 0 k E
A diSsertation submitted at La Trobe University in partial fulfillment of the requirements for a B.Sc (Hons.) Degree 1985.
ABSTRACT
A study of the petrology, geochemisty and field relationships of
sedimentary, volcanic and plutonic rocks from the Yeoval district has
revealed a complex and varied geological history. Volcanics derived from
the nearby Molong Volcanic Rise were dposited with marine sediments in the
Middle to Late Silurian. They were then deformed during the Bowning
Orogeny.
Intrusion of the Naringla Monzodiorite occurred in the Early Devonian.
This is a heterogeneous body ranging in composition from pyroxenite
cumulates to microgranodiorite and dacite porphyry. The latter two rock
types are the host to disseminated porphyry copper style mineralisation.
Co-genetic
prior to
basaltic andesites were erupted above the
final crystallization of the monzodiorite.
monzodiorite body
The pyroxenites,
pyroxene bearing basaltic andesites and pyroxene bearing monzodiorites
formed by crystal fractionation of augite from a primitive magma of
basaltic andesite composition. The chemical variations in the Naringla
Monzodiorite (apart from the low-Si rocks) is thought to have been derived
by chemical fractionation of hornblende, plagioclase and possibly biotite,
although magma mixing and restite unmixing cannot be ruled out as formation
processes.
Hydrothermal alteration associated with the porpyritic intrusions has
produced propylitic mineral assemblages in most of the rocks of the
Naringla Monzodiorite. Alteration is most intense around the copper
deposits, and occasional albitic alteration is developed. Alteration is of
a weak and patch nature, and lateration zones are generally not developed.
A small body of gabbro that outcrops adjacent to the Naringla
Monzodiorite is probably unrelated due to chemical inconsistencies between
the two units.
The Ohley Adamellite intruded during the Middle Devonian, and contact
metamorphosed the gabbro and a body of quartz-hornblende-diorite. The
intrusion of rhyolite dykes into the Naringla Monzodiorite may have been
related to the emplacement of the adamellite.
The final geological event in the area was the emplacement of small
basic dykes along east-west joints in the Naringla Monzodiorite. These
dykes are of probable Tertiary age.
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CHAPTER 1
1.0 1.1 1.2 1.3 1.4
CHAPTER 2
CONTENTS
INTRODUCTION
Introduction Location and Methods Employed Physiography Previous Work Geological Setting
1.41 Introduction 1. 42 1.43
Forbes Anticlinoria! Zone Cowra-Yass Synclinoria! Zone
SILURIAN VOLCANICS AND SEDIMENTS OF THE COWRA-YASS SYNCLINORIAL ZONE
2.0 2.1
2. 2 2.3 2.4
Cudal Group Description of Rock Units
2.11 Yullundry ~ormation 2.12 2.13
Canowindra Porphyry Hanover Formation
Regional Metamorphism Contact Metamorphism Conclusions
CHAPTER 3 INTRUSIVES AND VOLCANICS OF THE FORBES ANTICLINORIAL ZONE
3.0 3.1
3.2 3.3
3.4 3.5 3. 6 3.7 3.8
Introduction Naringla Monzodiorite
3.11 Basaltic Andesites 3.12 3.13
Gabbro Monzodiorite
3.14 Porphyritic Microgranodiorite 3.15 Porphyritic Dacite Dykes
Quartz Hornblende Diorite Obley Adamellite
3.31 Two Feldspar Adamellite 3.32 One Feldspar Adamellite 3.33 Microadarnellite 3.34 Aplite 3.35 Microadarnellite Dyke
Boxleigh Park Adamellite Rhyolite Rhyolite Dykes Basic Dykes Young Granodiorite
Page No.
1 2
4 7 7 7 7
8
11
11 11
13 15 19 20 21
22 22 22 26 29 35 38 39 41 41 42 42 43 45 45 47 48 52 52
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CHAPTER 4
4. 0 4.1
4.2
4. 3
CHAPTER 5
5.0 5.1 5.2
5.3
5.4
CHAPTER 6
6.1
6.2
ALTERATION
Introduction Pervasive Alteration
4.11 Propylitic Alteration 4.12 Sericitic Alteration 4.13 Albitic Alteration 4.14 Chloritic Alteration
Vein Alteration 4.21 Goodrich Deposit 4.22 Yeoval Prospect 4.23 Conclusions
Alteration Minerals 4.31 Chlorite 4.32 Albite 4. 33 Epidote 4.34 Prehni te 4.35 Sericite
MINERALIZATION
Introduction Goodrich Mine Disseminated Deposits
5.21 Yeoval Prospect 5.22 Porphyry King Prospect 5.23 Goonoo Prospect 5.24 Yeoval East Prospect
Shear Zone Deposits 5.31 Cyclops Mine 5.32 Suntop Mine 5.33 Southern Mineralized Belt 5.34 Northern Minera~ized Belt
Vein Deposits 5.41 Freehold Prospect 5.42 Timby Hills Mine
GEOCHEMISTRY
Geochemistry of the unaltered rocks 6.11 Geochemical Classification 6.12 Major Element Geochemistry 6.13 Trace Element Geochemistry
Discussion
Page
54 54 56 57 56 59 61 61 63 66 66 66 67 67 67 66
69 69 71 71 73 73 74 74 74 74 76 76 77 77 77
76 79 79 61 63
No.
CHAPTER 7
7.0 7.1 7. 2 7. 3 7. 4
CHAPTER 8
8.1 8.2
8.3
8.4
GEOCHRONOLOC::Y
Introduction Naringla Monzodiorite Ohley Adamellite Basaltic Andesites Discussion
DISCUSSION
Relationship between the Yeoval Rocks Petrogenesis
8.21 Fractional Crystallization 8.22 Magma Mixing 8.23 Restite Model
General Model for the Formation of Porphyry Copper Deposits Formation of the Yeoval Deposits
8.41 Yeoval Prospect 8.42 Goodrich Deposit 8.43 Cyclops Deposit
90 90 92 93 95
9B 99 99
100 100
101 102 104 104
105
ACKROWLEDGEMBHTS
I would like to thank Dr. T.A.P. Kwak, my supervisor, for his help,
advice and for financial assistance throughout the course of this project.
Special thanks to Dr. C.M. Gray, who freely gave his time for
discussions and criticisms, and for reading part of this thesis.
Thanks are also extended to the staff of the Geology Department who
were so helpful with their advice - Dr, R.C. Price, Dr. P.D. Fleming and
Mr. B. Salton.
David Steele and Peter Jackson are thanked for numerous discussions on
various aspects of this thesis.
was greatly appreciated.
I. McCabe and G. Holm's technical advice
Gratitude is extended to Mr. K. Barker for allowing me access to the
Goodrich Mine, and for many handy hints in the field. Thanks also to Bill
Fey for providing accommodation for much of the stay,
Miss H. Butt has assisted with many aspects of the preparation of this
thesis and I am forever indebted to her.
Profuse thanks to my sister Julie Pilarski, who bravely offered to
help, and persevered with patience in the typing of this report. Thanks
also to Mum and Dad for everything, and thanks for various reasons to G,
Lightbody, A. Butt, A. Capellazzo and P. Malaka.
Finally, I would like to thank my co":...sufferers; J. Schusterbauer, I.
Finlay, s. Reeves, I. Manton, G. Douglas and M. Nicholson for making this
year so interesting, and a special thanks to Michael Hartley and Anthony
Manini who assisted with transport during certain parts of this year.
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CHAPTER 1 - INTRODUCTION
1.0 Introduction
Most porphyry copper deposits are Mesozoic or Cainozoic in age and
appear to be intimately related to subduction processes. These deposits
occur both in island arc settings and in mountain belts at active
continental margins, Porphyry style deposits are also found in Palaeozoic
and Precambrian rocks, although these are rarer than the more contemporary
deposits. In the Palaeozoic Lachlan Fold Belt in New South Wales, small
uneconomic copper-gold deposits that exhibit porphyry copper style
characteristics occur. These deposits are Siluro-Devonian in age.
At Yeoval in central N.S.W., a monzodiorite pluton contains a number
of small copper-gold deposits which exhibit some of the characteristics of
porphyry copper deposits. The monzodiorite is of an Early Devonian age,
and has intruded moderately deep water Silurian sediments. In the Late
Devonian, a large adamellite pluton has intruded the monzodiorite and
sediments.
The aims of this thesis are:
(1) To study in detail the mineralization and alteration associated
with the copper-gold deposits in the monzodiorite and the
surrounding country rocks, with particular emphasis on the
Goodrich deposit. (See figure 1).
(2) To compare and contrast the Goodrich deposit with the well
documented Yeoval prospect north of Yeoval. (See figure 2).
(3) Analyses of relationships between the dykes and plutonic rocks.
(4) A study of the variations within the monzodiorite in the hope
that a petrogenetic model can be obtained.
( 5) Comparison of the Naringla Honzodior'ite with the Young Grano
diorite, and typical suites from the Lachlan Fold Belt.
(
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1.1 Location and Methods Employed
The Naringla Monzodiorite is an elongate NNE-SSW trending body with a
strike length of some 22 kilometres and a maximum width of 7 km. The
pluton is centred on the small town of Yeoval in central west N.S.W. (see
figure 1). Yeoval is located about 50 km south of Dubbo, 80 km north-west
of Orange and 370 km north-west of Sydney (see figure 1.1).
Sealed roads connect Yeoval with Wellington to the east and Molong to
the south-east, and unsealed roads connect Yeoval with Parkes, Peak Hill
and Oubbo (see figure 1.1). Farm tracks are present over most of the
mapping area providing good access. The exception is the heavily timbered
adamellite ridges on the western side of the area, where access proves to
be more difficult. Outcrop is generally poor, and is restricted to low
lying hills, The exception is the adamellite, which outcrops beautifully
in a prominent ridge around the north, west and southern margins of the
monzodiorite. The boundary between the adamellite and monzodiorite is well
defined and was mapped by ground traverses and air-photo interpretation.
Weathering is a problem in the rest of the mapping area, and contacts are
almost always unexposed. Because of this, outcrop mapping was conducted
and the contacts have been inferred. Mapping was done directly onto aerial
photographs, and this was then transferred to a 1:50,000 base map from
which the final map was constructed.
Detailed outcrop mapping has been conducted around the Goodrich Mine
and the Yeoval Prospect (see figures 2 and 3 respectively). Mapping was
done directly onto enlarged aerial photographs. These were then enlarged
again to produce the final map. Sketch maps were prepared of the Yeoval
and Cyclops mine areas (see figures 4 and 5 respectively).
Geochemical sampling was conducted mainly around the Goodrich mine
{figure 2). Due to the pervasiveness of the alteration, the samples
collected were the least altered representative samples found. Samples
were also collected of the most altered rocks for the purpose of
determining the chemistry of alteration.
Surface prospecting for scheelite was conducted at night using an ultra
violet lamp.
3
(
Figure 1.1 - Locality Diagram
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1.2 Physiography
Yeoval is part of the Western Slopes region of New South Wales. The
Western Slopes region is composed of low, rolling hills that form part of
the catchment area for the Macquarie River. The Little River drains the
western side of the area, and the Buckinbah Creek drains the eastern side.
These converge in the north of the area. A number of small creeks
seasonally drain the central portions of the mapping area before flowing
into the Buckinbah Creek.
The topography is dependent on the underlying units. The monzodiorite
forms gently sloping and flat lying plains. It is surrounded on three sides
by prominent, heavily timbered adamellit? ridges. To the east, the
volcanics and sediments of the Cudal Group outcrop very poorly in a
generally flat lying terrain.
The average rainfall is 58cm per year, this being evenly distributed
throughout the year. The climate is temperate and the average summer
temperature is 25"-30" celsius, although the average temperature at the
time of mapping was 35"C, and the area was in the grip of a serious
drought.
Sheep and cattle are grazed over the flat lying areas, and various
crops are grown. In the adamellite ridges there is some grazing of live-
stock but the terrain is too rugged and the soil too poor for the growing
of crops.
1.3 Previous Work
Gold was first discovered at what is now known as the Goodrich mine in
1868. Mining of gold and copper was subsequently carried out until 1886
when a fatality during mining caused the closure of the open pit. The mine
was reopened in 1903 and worked until 1909. During this period, numerous
other areas around Yeoval were mined but, apart from the Goodrich deposit,
production was very low. In his report on the copper mining industry of
New South Wales, Carne (1908) reported the presence of copper in the Yeoval
district. Some prospects were worked up until the late 1930's (e.g.
Freehold Prospect), but all work ceased with the advent of World War Two.
(
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More recently, prospecting has been carried out on a small scale around the
Goodrich deposit and other old shafts by a local prospector, Mr. Kevin
Barker. He was the first person to find scheelite at the Yeoval prospect
in the late 1970's.
The first geological study of the area was carried out in 1945 by
Basnett and Colditz. They described the regional geology between
Wellington and Yeoval, and their work was followed on by Maggs {1963) in
his thesis on the stratigraphy between Yeoval and Manildra. He defined the
sediments and volcanics east of Yeoval as belonging to the Manildra group.
The term Manildra Group was subsequently discarded by Brunker et al (1970)
and the Cudal group to the east was extended to include all of the original
Manildra Group. The Cudal group was then redefined by Pickett et al (1982)
to include further formation that were first classified by Maggs (1963).
In the mid 1960's, the Geological Survey of New South Wales conducted
four studies of the Goodrich Mine (McClatchie and Dickson (1963), Ringis
and Webster (1964), Ringis and Kennedy (1964), McManus and Loudon (1966)).
The most important finding of these reports was that the mineralization at
the Goodrich Deposit is controlled by a steeply dipping arcuate narrow
structure.
shear.
This structure has been truncated by an easterly striking
A Ph.D. thesis on the rocks of the Yeoval Batholith was completed by
Brian Gulson of A.N.U. (1968). He was the first to delineate what he
described as a dioritic phase of the Yeoval Batholith as being a separate
entity from the main adamellite phase of the batholith. His isotopic
studies (Gulson and Bofinger 1972), showed a clear time difference of 40
million years between the intrusion of the diorite and intrusion of the
main adamellite phase of the batholith. From this he concluded that there
was no genetic relationship between the adamellite and the diorite. Gulson
classified the rocks of the Yeoval Batholith by their chemical composition.
He found that the bulk of the diorite was actually a high-K diorite (Gulson
1972), which differs from normal diorites only in a higher potassium and
related trace elements content. High-K diorites are believed to be the
coarse grained equivalents of the high-K andesites (Gulson et al; 1972).
However, Gulson's high-K diorites have since been identified petrologically
as granodiorites (Patterson et al 1983, Ambler 1979), and as monzodiorites